The present invention relates to a safety device for an extracorporeal blood treatment machine and, more particularly, to such a device for monitoring the ultrafiltration rate for a hemodialysis, hemofiltration or hemodiafiltration machine. The invention also relates to a method of increasing the safety of an extracorporeal blood treatment machine.
Various methods of surgical purification or treatment of blood are used on patients with chronic renal failure. Such methods are used to remove undesirable substances that typically pass through a healthy person""s urine. The methods are also used to remove excess body fluid in the patients. Diffusive mass transport is the predominant mechanism in hemodialysis (HD), while convective mass transport through the membrane is involved in hemofiltration (HF). Hemodiafiltration (HDF) is a combination of both methods.
Removing excess body fluid by ultrafiltration is an essential component of dialysis therapy. With the standard form of therapy today, either a fixed ultrafiltration rate (UFR) or a fixed time characteristic of the ultrafiltration rate (UF profile) is selected. The ultrafiltration rate and the ultrafiltration volume during a treatment affect cardiovascular regulation and the fluid compartment of the patient""s body. Therefore, ultrafiltration is critical for safety. With uncontrolled withdrawal of fluid at the rate of several liters/hour (L/h), a significant decrease in blood volume may be reached after a few minutes. Such a decrease may result in a severe drop in blood pressure necessitating intensive care measures.
The ultrafiltration rate UFR of a dialyzer increases in proportion to the prevailing transmembrane pressure TMP, where transmembrane pressure is the pressure difference between the average pressure on the blood end and the average pressure on the dialysate end. The ultrafiltration coefficient indicates the amount of fluid removal (ultrafiltration amount) that can be achieved per hour and per mm Hg transmembrane pressure (dimension: mL/hxe2x80xa2mm Hg). The ultrafiltration coefficient UFK and the desired fluid removal thus yield the transmembrane pressure TMP to be established.
To increase the patient""s safety, existing blood treatment machines typically have a safety device which monitors the transmembrane pressure during the blood treatment to determine whether it exceeds an upper limit or drops below a lower limit, said limits defining a window of safety. An alarm is triggered on leaving this window. Reaching the upper limit may indicate blockage of the dialyzer, and reaching the lower limit may indicate a leak in the fluid system. In a blood treatment with a constant ultrafiltration rate UFR, the window of safety is set manually before starting the blood treatment on the basis of measured transmembrane pressure values. However, if the ultrafiltration rate changes during the blood treatment, the window must be readjusted. Known blood treatment machines allow for a stepwise ultrafiltration profile to be selected so that the window may be automatically readjusted on the basis of new measured values. However, centering the window after a sudden change in ultrafiltration rate generally takes more than one minute with such known devices.
If the ultrafiltration rate is to be regulated continuously, the window must be readjusted constantly. However, if centering the window takes more than one minute, the protective function may be eliminated entirely or at least in part.
German Patent Application No. 34 16 057 A1 describes a hemodialysis machine having a program-controlled device for producing a variable composition of the dialysis fluid. A check is performed during the treatment to determine whether the conductivity of the dialysis fluid is within a predefined window. The conductivity monitoring window is shifted with a change in the composition of the dialysis fluid thereby necessitating readjustment.
German Patent No. 42 39 937 C2 discloses a monitoring a device for hemodialysis equipment intended to prevent ultrafiltration rates that would be harmful to a patient. The disclosed device monitors transmembrane pressure to determine the existence of an undesirable ultrafiltration rate. However, the disclosed device is not suited to reliably monitor transmembrane pressure when the ultrafiltration rate is not constant.
The present invention is designed to overcome the deficiencies of the prior art discussed above. It is an object of the present invention to create a safety device for an extracorporeal blood treatment machine that is adapted to allow transmembrane pressure to be accurately monitored even with a variable ultrafiltration rate.
It is a further object of the invention is to provide a method of increasing the safety of an extracorporeal blood treatment machine.
In accordance with the illustrative embodiments, demonstrating features and advantages of the present invention, there is provided a safety device for an extracorporeal blood treatment machine of the type that includes an exchange unit divided into first and second chambers by a semipermiable membrane and an ultrafiltration device adapted to remove ultrafiltrate from the exchange unit at varying ultrafiltration rates. The safety device includes a measuring device for measuring transmembrane pressure TMP of the exchange unit, an establishing device for establishing an upper limit L1 and a lower limit L2 for a transmembrane pressure TMP, and a comparing device for comparing a measured transmembrane pressure TMP with the at least one of the upper limit L1 and lower limit L2. The establishing device includes a computer unit for calculating a transmembrane pressure at a preset ultrafiltration rate and for calculating the upper and lower limit L1 and L2 transmembrane pressures on the basis of the calculated transmembrane pressure. The comparing device is in electrical communication with the measuring device. The present invention also relates to a method of increasing the safety of an extracorporeal blood treatment machine.
Other objects, features and advantages of the invention will be readily apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the drawings.